The invention relates to a system for controlling the thrust nozzle adjustment of two cycle gas turbine jet propulsion engines. More specifically, the invention relates to a method and to an apparatus for said adjustment, whereby the opening angle of the thrust nozzle is adapted to the instantaneous operating condition of the jet engine.
A system in which the present invention may be practiced comprises a low pressure compressor driven by a low pressure turbine and a radially inwardly situated compressor for the first flow cycle or circuit and a radially outwardly situated compressor for the second flow cycle or circuit. The system also comprises a high pressure compressor, which is driven by a high pressure turbine. A combustion chamber is located upstream of the high pressure turbine. An afterburner is supplied with the turbine gases from the first, hot flow cycle and with compressed air from the second, relatively cold flow cycle. The system is equipped with multiple shafts.
Adjustable thrust nozzles for varying the effective exit surface for the thrust jet are primarily used in gas turbine jet propulsion plants with afterburning, because this operating condition requires an increased flow rate or throughput and higher gas temperatures. The adjusting of the thrust nozzles is accomplished by an open loop or closed loop control as a function of various parameters, in particular as a function of the turbine pressure gradient or differential, which represents a proportional measure for the instantaneous power of the engine.
The known types of closed and open loop controls for the thrust nozzle adjustment do not take into account the full scope of the aerodynamic and thermodynamic complexity of a gas turbine jet propulsion plant.
Special problems occur in connection with engines of this type having the highest specific power, having regard to the fastest load changes, particularly in aerial combat calling for high velocity changes and large flight altitude differences. These problems must be solved with a high safety factor. In addition, a small specific fuel consumption is required substantially for flight conditions, especially during cruise flight.
The axial compressor substantially always used in high power propulsion plants has great advantages, but it only has a relatively narrow stable operating range. In addition, the tuning of the gas turbine with an axial compressor still causes great difficulties even today. An engine construction is required, which allows an operating of the gas turbine over a very large load range without compressor pumping or surging. Due to this phenomenon, the air flow between the individual compressor stages separates or is interrupted, which may lead to a considerable power reduction of the engine even to the extent of a complete collapse of the flow process. Several measures have been developed in order to avoid the feared compressor surging or pumping. First, it is possible to adjust or increase the effective cross section of the air intake in the supercritical operating condition. Secondly, the effective opening cross section of the thrust nozzle may be varied in order to avoid a detrimental reverse or feedback effect on the aerodynamically sensitive axial compressor or compressors. In addition it is possible especially under partial load operation, to adjust the guide blades of the compressor and/or to blow off some of the compressed air between the individual compressor stages, which however, causes a reduction in power. The dividing of the axial compressor of multiple shaft engines of the type mentioned above into several compressor groups, which operate with different rpms also serves particularly to prevent the compressor surging or pumping and to further extend the surging limit.
The operating performance characteristics of a gas turbine jet engine or propulsion plant with afterburning on the other hand, is determined by the quenching limit of the afterburner. The afterburner is blown out by too large flow mach numbers, at which a stable combustion may no longer occur. Hence, the exceeding of this limit is also to be avoided.